The present invention relates to a hybrid continuously variable transmission.
U.S. patent application Ser. No. 07/330,918 filed on Mar. 31, 1989 by Abo et al., now U.S. Pat. No. 4,895,552 issued on Jan. 23, 1990, discloses a hybrid continuously variable transmission. This previously proposed transmission has two power trains between an input shaft and an output shaft, which are selectively rendered operable to establish a desired reduction ratio between the input and output shafts.
When it is rendered operable, a first one of the two power trains provides a single reduction ratio which is fixed. A second one or the other of the two power trains, when it is rendered operable, provides a reduction ratio which is continuously variable over a predetermined range. More specifically, the known hybrid continuously variable transmission including a gearing mechanism combined with a V-belt type continuously variable transmission mechanism. A first clutch, namely a low clutch, and a second clutch, namely a high clutch, are provided. When the low clutch is engaged with the high clutch disengaged, an engine power is delivered by the gearing mechanism, while when, with the low clutch kept engaged, the high clutch is engaged, the engine power is delivered by the V-belt type continuously variable transmission. A shift from the power delivery state by the gearing mechanism to the power delivery state by the V-belt continuously variable transmission mechanism is effected by engaging the high clutch. Although the high clutch is engaged with the low clutch kept engaged, a one-way clutch is released to interrupt delivery of power from the gearing mechanism to the output shaft. Thus, this shift is made smoothly owing to the action of the one-way clutch.
The V-belt type continuously variable transmission mechanism includes a driver pulley with a driver pulley cylinder chamber, a follower pulley with a follower pulley cylinder chamber, and a V-belt drivingly interconnecting the driver and follower pulleys. For controlling the continuously variable transmission, a hydraulic fluid pressure acting within the driver pulley cylinder chamber is varied by a shift control valve. The shift control valve includes a spool which is operated via a shift operating mechanism by a shift motor in the form of a stepper motor under the control of a microcomputer controlled unit. With this arrangement, the hydraulic fluid pressure within the driver pulley cylinder chamber assumes different values each corresponding to one of different positions which can be taken by the stepper motor. This hydraulic fluid pressure variable by the stepper motor is admitted to the high clutch.
As mentioned above, the high clutch is activated by the same hydraulic fluid pressure acting within the driver pulley cylinder chamber and thus controlled by the stepper motor. In other words, a rate of increase in the hydraulic fluid pressure admitted to the high clutch is determined by a speed at which the stepper motor rotates. Thus, it is very difficult to increase the hydraulic fluid pressure admitted to the high clutch at a rate gradual enough to suppress a shift shock to a satisfactorily low level.
If, during a panic braking, a quick downshift and a quick disengagement of the high clutch are required, it takes a considerable time until the high clutch is disengaged since the hydraulic fluid pressure acting within the high clutch does not drop to a sufficiently low level until the stepper motor rotates in a downshift direction to a predetermined position. Since it is subject to slip during this transient period, the V-belt wears at a quick rate, resulting in degraded endurability and shortened service life.
An object of the present invention is to improve a hybrid continuously variable transmission such that a clutch which controls a shift between two power train drive states is engaged without any substantial shock and is disengaged quickly when so required.